[Java][RestClient] Enable access token refresh

[wandi34]

Similar to other Java Client libs (e.g. RestTemplate PullRequest), RestClient should also allow setting a bearerToken supplier instead of just a static string.

I used the given RestTemplate PR as a template for this PR.

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wandi34]

Hi wing328, thanks for the hint. I fixed the signature, now its signed correctly.

[wing328]

thanks for the PR, which has been merged.